The mechanism proposed for the catalytic action of the pancreatic enzyme carboxypeptidase A on esters and peptides involves the postulated intermediacy of anhydrides formed between the carbonyl group of the substrate and the carboxylate group of Glu-270. In experiments with an ester substrate at sub-zero temperatures in mixed aqueous-organic solvents, we have detected an acyl-carboxypeptidase A species for the first time. In nucleophilic trapping experiments with hydroxylamine which are nearing completion, we have found that hydroxamate is incorporated at a glutamate residue, presumably Glu-270. Employing low temperature methodology for the detection of acyl-carboxypeptidases, we intend now to concentrate heavily on studies of the roles of metal ions present at the active sites of various metallocarboxypeptidases such as the Co ions, Ni ions, Mn ions, Cd ions, Hg ions and even the Cu ions species. Also, we have developed a method suitable for the large scale preparation of bovine carboxypeptidase B, and we intend to carry out a similar investigation of the mechanism of action of this enzyme.